Upgrade project to .NET 6 with C# 10

[oliverbooth]

This will inherently (yet not inadvertently) remove Unity support.

• [x] Filed scoped namespaces
• [x] Implicit usings
• [ ] Add extension methods for generic math
• [x] Re-add support for Unity

[oliverbooth]

Interfaces with static abstract members is a preview feature, which enabling would require consumers of X10D to also enable preview features.

I'm not entirely happy doing that, therefore this issue will be on the backburner for the forseeable future

[oliverbooth]

Unity support has been reintroduced as of 3.1.0-nightly.89, with multi-target build. X10D now targets both .NET 6 and .NET Standard 2.1